Sensing elements in polymer-coated vapor sensors are often surface acoustic wave (SAW) devices. To overcome selectivity limitations, arrays of SAW sensors are used, employing a micro-sensor system format which has pre-concentration, chromatographic column resolution and array detection stages [1]. One reason for this choice of SAW devices is the higher sensitivity afforded by these typically higher frequency devices. Thickness shear mode (TSM) devices, also called the quartz crystal microbalance (QCM), have been employed in many analytical applications, and as commercial thickness monitors in deposition equipment, however, their use in vapor and gas sensing is not common, although, some electronic nose type systems do employ them in odor sensing, which is essentially a vapor sensing application [2-5]. Some advantages of the TSM devices include simpler electronics, better baseline stability, less involved design and fabrication, and well-developed response models.
Most TSM applications so far have utilized devices in the 5-20 MHz fundamental resonance frequency range. Commercial resonators have been manufactured routinely for some time by thinning quartz plates by chemical milling to oscillate at fundamental frequencies of a little over 100 MHz and stable oscillators at fundamental and overtone frequencies are readily available. However, such higher frequency devices have rarely been tested in sensing applications and never in gas phase applications [6]. A concern in utilizing TSM devices of greater than about 20 MHz fundamental frequency in sensor applications is the fragility resulting from the thin piezoelectric material that is necessary for obtaining higher fundamental frequencies. To provide mechanical stability, devices are fabricated by surrounding a thin oscillating region with a thick outer ring. In addition to the fragility, such thin plates (of the order of 2 micron for a 100 MHz quartz device) could lead to baseline stability and consequent limit of detection issues. Finally, the active sensor area is typically reduced by design limitations on the ratio of the oscillating region to the outer ring, and the absolute value of the oscillating region, leading to sensor performance issues that cannot be predicted easily.